Synaptic connectivity -- that is, synapse number and strength -- determines the computational power of neural networks. This begs the question; how are synaptic connections made, maintained and modified to accomplish the computational power of the mammalian brain? To address this question, I will image presynaptic proteins tagged with the green fluorescent protein (GFP), and its photoactivatable derivative (paGFP), in vivo and measure the following fundamental parameters as a function of development and experience: 1) How do axons branch and what fraction of new axonal branches are maintained into adulthood? 2) What fraction of presynaptic terminals formed during development are maintained into adulthood? 3) Does synaptic activity correlate with synaptic maintenance? These experiments will represent firsts in the characterization of cortical neurons in vivo during development and in the tracking of individual protein populations in vivo -- methodologies which will bridge the gap between molecular and cellular neurophysiology.